Automatic frequency control system



Jan 4, 1949-- G. FREDENDALL T V2,453,156

AUTOMATIC FREQUENCY CONTROL SYSTEM TTOP/VEV Jan. 4, 1949.

Filed July 29. 1944 AUTOMATIC FREQUENCY- CONTROL SYSTEM G. L. FREDENDALL 2,458,156

` 2 Sheets-Sheet 2 CONT/2u Wam' FaQ/w Patented Jan. 4, 1949 idn'iixi OFFICE AUTOMATIC FREQUENCY CNTROL SYSTEM Gordon L. Fredendall, Feasterville, Pa., assigner to Radio Corporation of America,

of Uelaware acorporation Application .lilly 29, 1944, Serial No. 547,230

(Si. o-36) 1 l Claim.

The present invention relates to' automatic frequency control or phase control circuits and particularly to novel apparatus for the synchronization of cathode ray deflecting circuits in television receivers or the like.

In some locations, generally near the edges of the service area of a television station, local interference or weak signals, or perhaps both, impair the accuracy of synchronization inv a television receiver to such an extent that the horizontal resolution is appreciably reduced. There may also be failure of interlace and even sporadic failure of synchronization. This invention aims to provide an improvement .in synchronizing action that will improve performance inthese marginal locations without introducing comprising factors. For signal conditions which are-at all satisfactory ior reception, substantially the maximum yhorizontal resolution. permitted by the television channel may be realized in a receiver embodying the present invention when allowance is made for unavoidable noise effects `seen in the image as beam modulation. Also, noise effects do cause tearing of the picture in horizontal strips. Appreciable improvements in lvertical synchronization is shown.

in accordance with this invention, these and other desirable results'are attained by providing improv'ementsinV previously known television receiving equipment in which synchronization of an oscillator'is to be obtained by employing an automatic frequency-'control' circuit responsive to any change'in the timerelationship 0f the synchronizingpulses with respect tosawtooth waves usedto control deflection;

A further object of the invention is 4to provide an improved means forI synchronizing the deflecting circuit or circuits of an image producing device.

A still further object of the invention is to provide an improved televisionv receiver.

ln accordance with the invention, a phase detector is coupled in a novelhmanner to a signal source.

Other and more specific objects of the invention Will become apparent andl suggest themselves to those skilled in therart to which the invention is directed upon reading the `following speciiication and claim in connection With the accompanying drawing in Whicht Fig. 1 is a circuit diagram of substantially the entire deflecting arrangement of ay television receiver embodying the invention; and

Fig. 2 is a schematic representation of Ya portion of a television-receiver embodying a modiiication of theinventiom..

Referring to Fig. 1 of the drawings, certain4 parts of a employed in previously known television receivers; l its functions and operation will not be described-4 in detail. rlhe output to the image producing device such as a Kinescope Usually, this connection will effectively terminate at the control grid of of the duplex type serves as a D. C. restorer and also as a sync pulse separator and amplifier. l The grid (52Y together with the. serves asv a diode. rectiiier.:

cathode iii and the cathode resistor 53 With its load resistor to perform the function `of a D. C. inserter since the control grid circuit of the image producing peaking circuits 55 to be rcpeatedzby wayof a condenser 58 on the control grid .69r 55. The grid and cathode elements lil and 62 rec-` tify the peaks of the synchronizingpulses which are applied to the cathode with'negative polarity,

and only the sync signals are passed by the condenser 55 to the grid 59. The pulses appearing on the grid E9 are present in the plate circuit of the Y a vnovel .mannenx hereinafter described, tothe automatic frequency controlled horizontal 0r vertical deflection cir-` plate 'H which is coupled in cuits.

The horizontal `deflection circuit ofthe image producing device is fed in the usual manner fromA The details of the horizontal saw-tooth generator 3l and the function and operation of the horizontal phase control 92 will now be described. The cathode 9i and anode 96 ofthe duplex. type tube 88 are elements of the usual blocking oscillator tube. The blocking oscillator includes in typical television receiver, modified-to incorporate features of this invention, areshownfrom the second detector of the receiver is indicated 4at 58.

the Kinescope. A. tube 59Sv device is coupled/through.; the resistor 53. The control grid S2 alsofcausesthe signals applied over a conductor 66 from the l of the tube addition a feedback transformer 91, a condenser 99, which is charged and discharged, and a speed control resistor |02 Which is in series with a resistor |03.

The blocking oscillator section of the tube 88 operates to produce pulses and is controlled from the phase control 32 in a manner to be described. The elements |03 and |38 of the tube 83 serve as a discharge tube to generate a saw-tooth of voltage by charging and discharging a condenser |69. The adjustable resistor serves as a Width control. The sawtooth voltage across the condenser ||J9 is applied through a condenser 2 to the grid ||4 of the high impedance horizontal output tube 89. The plate circuit of this tube includes the primary ||6 of the transformer 36 which also serves as a coupling impedance for applying voltage pulses to a iiiter composed of a resistor |2| and a condenser |22 by way of a conductor ||9 and a condenser |23. This resistor |2| and condenser |22, in combination, produce a Wave of saw-tooth form from the puls-es derived from the output of the tube 89, the latter being of such character that the saw-tooth current is produced in the horizontal deilecting coil of the Kinescope. The condensers |23 and |23 correspond to the condensers 23 and 26, respectively, of Fig. l of a co-pending application of the present applicant jointly with Alfred C. Schroeder, Serial No. 547,355 and led July 81, 1944. Resistors |74 and |75 correspond to the resistors 34 and 33 respectively of Fig. l of said application. The filter' constituted by the components |3|, |32, and |30, corresponds to the lter 39 of Fig. 1 of the said application.

The lter-resistor |33 has a critical value, for

each receiver of a given design in which it isv used. When this resistor is too large, th-e sides of the image become scalloped, and when it is too small, the image as a Whole vibrates in the horizontal direction. There is an intermediate range of values for Which the image is correct.

A phase inverter tube |33 is coupled to the output from the plate 1| of the tube 59 through a coupling condenser |34. The coupling condenser |34 in conjunction with the grid resistor |36 of the tube |33 serves as la diiierentiating circuit to separate horizontal sync pulses from the output of the tube 59. The pulses which are applied to the grid |38 are of short time duration which is desirable for application to the phase control 92.

A phase inverter tube Ml is coupled to the output of the tube 59 through a coupling condenser |43 and a resistor |34. The resistor |43, in conjunction with a cond-enser |46, serves as resistor IGI. As a consequence of capacitance eilects in the tube |33, the resistors |6| and |62 are not necessarily oi equal value. p

The condenser |2l| serves to couple the plate oi the tube |33 tothe cathode |64 of one of the diode sections. The corresponding anode |36 is joined to the cathode |67 of the other diode section, and this connection, indicated by reference character serves as the output terminal of the phase detector 92. The D. C. return paths for the diode sections are provided by resistors |14 and |75, having a common terminal which is grounded as shown, thereby providing a balanced arrangement for the phase detector. The anode |69, corresponding to the last-named cathode, is coupled by the condenser |26 to the cath- W ode ofthe tube |33.

an integrating circuit to obtain separation of the vertical sync pulses Which are to be applied to the vertical phase control designated generally by a reference character |38. Inasmuch as the action of the vertical sync circuits is similar to the action of the horizontal sync circuits, a detailed description will be given in connection With tube |33, and it Will be .understood that this description applies equally to the tube IM.

Pulses appearing on the grid |38 of the tube |33 are applied in opposite phase by way of the previously mentioned oondensers |24 and |28 to a phase detector comprising a pair of diodes or, as shown by way of example, a duplex diod-e |59. Coupling is effected in a manner to provide positive pulses on the plate and negative pulses substantially of equal amplitude on the cathode of the tube |33. Equality of amplitude is readily otbained by selecting a proper value .for the plate The diode sections of the duplex diode |53 function as peak detectors by selecting Values for the resistors |34 and |75. The discharge paths of the condensers |26 and |215 have a relatively long time constant.

A D. C. amplier |82 amplies the control signal from the phase detector Which appears at the point This amplifiedv control signal is applied to the grid resistor |03 of the blocking scillator triode section of the tube 88. This outs put is in the form of a signal Whichslowly Varies when noise is present. The variation in this signal is added to or subtracted from the potential normally appli-ed to the grid |83, this normal potential being variable at Willwithin chosen limits by the speed control resistor |32.

The operation of the receiver of Fig. l With the horizontal deflecting circuit arrangement as shown, in accordance with the invention, will now be described. Sync signals of opposite polarity with respect to ground are provided by the phase inverter tube |33 by Way of the condensers |24 and |26. These sync signals of opposite polarity are superimposed on the saw-tooth Wave derived from the tube 89 through resistors |83 and |88. The sync signal which is applied through the condenser |2| to the p-late |69 of the tube |53 is in the positive sense, while the sync signal applied by Way of the condenser |24 to the cathode `|64 is in the negative sense. The saw-tooth Waves on the plate |69 and the cathode |64 are in phase and the time relation is such that the sync signals are positioned on the return or re-trace portion of the saw-tooth. In normal operation, assuming that disturbances such as noise are absent,

and with a pre-determined normal setting of the l control |92, the sync signals are substantially at the middle of the re-trace. Any change in the phase relation of the saw-tooth Wave and sync pulses will cause an increase in the peak amplitude of one composite .signal and a decrease in the peak amplitude of the other composite signal, these composite signals being the saw-tooth combined with one set of sync pulses extending either in a positive or negative sense.

The voltages across the resistors I'lll and |15, that is to say, on the diode side of the condensers |24 and |26, include 'the D. C. voltage inserted by the peak rectifier action of the diode Sections of the duplex diode |59. The peaks of the sync pulses are held at the same reference level, which may conveniently be zero.

In the signal appearing at the point the sync pulses are effectively balanced out, leaving a signal containing only the saw-tooth wave components. The saw-tooth component is filtered out by the previously mentioned filter composed of the elements, |36 to |32, and a D. C. control potential is obtained whichis not a .fluctuating saw-tooth Wave but is a very slowlyrvarying signal when noise is present.. This .controllpotential is amplified by the amplifier tube |82. Injthe illus' trative example, the average potential "eXistin'g' .5'v

between the saw-toothwaves withthe'peaks'of the sync pulses at the same reference level' is zero at the instant when the sync pulses occur", If ai; small phase displacement occurs betweenth'e'syri'c` pulses and the saw-tooth waves, the peaks' Vofjthe.` sync pulses continue to be maintained at vzero po-A tential, for example, but the average potential atH the point lll is lowered'or raised, depending'pn whether the pulse moves to an advancedior to Ya4 retarded position in time relative to.th'e.saw

tooth wave. The feedback action issuch' that 'the change in speed of the oscillator tends tojreduce'; the magnitude of thephase discrepancy between' a saw-tooth cycle anda pulse that gave 'rise'to' the increment in the control voltage.

The equilibrium position of the pulses isa func; tion of the settings of the speed control, for example, speed control 102; but is constrained to li'e somewhere on the return time of the saw=tooth2 The suggested normal location given above was in the middle of the returntizneof-the saw-tooth. When the amplitude of noise is sufficiently great, some noise peaks tend to beheld at'zero-potential by the action of the diodes ofy the l"duplex diodel tube |59. The potential at the point tends to fluctuate about zero, if the Vdistribution of the noise is random. Under such circumstances, the D. C. component is zero.' Itis 'important Lhereto note that the waveforms of noise and saWj-tooth do not appear at 'the'junction of the diodes as dise tributing components of the" control' voltage; Variations in the` difference" between the-in-4 stantaneous A. C. axesv of the'Vv two saw-'tooth waves'would appear atthe'point l|1| in the :ab= sence of the filterlBU-iEZ. The function ofthe nlter is Athe removal ofthe higher frequencycome ponents of the control voltage and the transmission only of the very low frequency components. In. the absence of noise, thelatter are atrueindicationthat a readjustment in the phase of 'the oscillator is necessary for the maintenance of accurate synchronism. Low frequency components are indications of a drift in circuit constants or small changes in the frequency of the transmitted sync pulses.

The vertical deflection circuit and vertical deflection control shown by Fig. 1 of the drawings are similar to and operate in the manner already discussed in connection with the horizontal deflection circuit. The adjustable resistor |90 serves as a size control and corresponds to the size control i l l. The adjustable resistor |92 serves as a speed control and corresponds in function to the adjustable resistor |02. The adjustable resistor |93 serves as a vertical linearity control. A condenser ll is charged and discharged to provide a voltage wave which controls the plate current of a tube i9?. This plate current in the primary of transformer |98 serves to drive a sawtooth wave of current through the vertical deflection coil (not shown).

Polarities relative to ground of the vertical sync pulses appearing at the vertical phase detector |99 should be such that positive pulses are applied to the plate 29d and negative pulses to the cathode 292. The limits of the value of resistor 29e, which is the damping resistor in the filter combination 2%, 298, and 204, in general need not be as restricted as the value limits of the corresponding resistance in the horizontal undery the influencefof f severe-N noise.

berXedVv bythe-'speed' of-h pull-in considered "desir,- able and the tightness of lock-in;desired.;Y

It is ydesirable inrthe' operationfof van automatic frequency controlled television received to main-y A- tain `vafreasonably constant plate voltage supply tubesy 881 295, and-'|91 when the gain control.` of theiLFamplier (not shown) is fe`ctive-v arrangementy fori insuring a filediMarcliflS;` 1944,` now. matured Number 2,454g? ReferringtoFi-g. 2'of the drawings, a portion.. sof a"te1e`visliorr'receiverk embodying the invention. in'm'odie'd form `is disclosed. Reference charac-- the.l primaryZ 4 of "Whichsis `V`furnished#with separated horizontal i and-'vertical sync `signals over a connection indie,- fcateollat-'215;k This connection 2l5-may correspondwithf-or'be-analogous to the circuit of the ofthe -tube-59 'of Fig. 1. Push-pull horie zontaleA and vverticat syncpulses required by the horizontal andyertical phaseldetectors -2||"ar1d"- 2 B are derived from the transformer 2 |2 by` meansfof-hig'hepassRe-C filter network 2|9 and ter= `2 2 "denotes a'y transformer,l

plate a lowpassR-C filter network- 22 0,

Tl'i'efhorizon-tal'sync controlwill first bede-v scribed'isi-nce; as uwas-'stated in connection with :FiglofV the drawings, the principal features of the vertical sync controlare analogous and there-- forea detailedl description of it need not be given.

Thefphase-detectorZ l1 comprises apair of diodes or;fin-the interest of economy,y a duplex diode 222V comprising s independent diode sections 223 and- 2245V The' high-pass lfilterwl-is designed inac-l cordance with Well-knownprinciples and comprises-twosections; each composed of a condenser and'resistor. Condenser 226 and resistor 22? provides sync pulses in the positivesense to the'diode section 2'23. Condenser 228 and resistor 229 provide a sync pulse in the negative sense to the diode section 224. Condenser` 233 corresponds in function and location to the condensr |26 of Fig. 1. Condenser 234 corresponds in function and 1ocation to the condenser |24 of Fig. 1.

Resistors 236 and 237 are grounded at their point of connection to cause the diode sections to serve as peak detectors. In order that a control signal representing the phase difference between sync signals and the saw-tooth deflection wave may be developed at the point 239, the saw-tooth deection wave is fed by way of a connection 24| to the zero ground potential point, provided at the junction of the resistors 229 and 221. 'The arrangement from the condensers 233 and 234 may be the saine as the arrangement shown on Fig, 1 of the drawings from the condensers |24 and |26 through the horizontal deflection transformer 86 shown on Fig. 1 of the drawings. The connection '24| corresponds to the connection I9 of Fig. 1. Inasmuch as the arrangement of Fig. 1 may be used, it is not believed that further detailed illustration of these features is necessary. The saw-tooth wave is derived, for example, from a discharge tube under control of a blocking oscillator tube which functions in the manner explained in connection with tube 88 of Fig. 1 of the drawings. The output of the discharge tube appearing in its plate circuit is supplied through |82f-'andf2ill (or thotwo'parts cfa. duplex tube, if one -is used as shown) andfthe:

varied. Ansi-J.l

reasonably.V V:constant-plate"voltage is shown in ithe applica:.

tionvof-"Gordonr LL FredendalL' Serial No. 526,287; into Patent the medium of a transformer to the horizontal defiecting coil (not shown). The output of the discharge tube may, it will be understood, be of a wave form which is suitable to provide a current wave of saw-tooth form in the horizontal deflecting winding. Y

The synchronizing peaks, applied in opposite sense by the connection 2M and the saw tooth wave voltage are combined in a manner previously explained in Fig. l of the drawings to provide a control signal at the point 239.

vThe principal difference between the vertical and horizontal control circuits in this modification lies in the design of the Ylter 220 which passes the vertical sync signals Since the details of the lter 229 conform to normal practice,and the choice of constants of the filter can readily be made by those skilled in the art,.a detailed description will not be given. It is to be `noted that the capacity arms of the positive and negative portions of the filter are grounded at 269. The saw-tooth voltage from the vertical discharge tube is applied over a connection 272 to each half of the phase detector by resistors 274 and 216.

As a consequence of the balanced nature` of both vthe horizontal and vertical control circuits in accordance with this modification of the Iinvention, there are no troublesome reactions between horizontal and vertical circuits.A Economy of design is also a feature of the invention in this form since only the single transformer 2| 2 is required for both the horizontal and vertical circuits. This feature of economy is of considerable importance in television receivers designed for household use which, to be available generally to the public, must be sold at a modest price.

Various alterations may be made in the present invention without departing from the spirit and scope thereof, and it is desired that any and all such alterations be considered within the purview of the present invention as defined by the hereinafter appended claim.

Having now described the invention, what is claimed and desired to be secured by Letters Patent is the following:

.An automatic frequency control system comv prising a pair of peak rectiers, a source of signals of sawtooth wave form, a source of separated pulses providing high and low frequency pulses, a transformer connected to said source of separated pulses, means for applying sawtooth wave form signalsto said rectiers with like polarity at each rectifier, means including a high pass filter for applying pulses `of high frequency to said rectiers from said transformer with opposite polarity at each rectifier, means for utilizing the output of said pair of peak rectiers for controlling said means for applying sawtooth wave form signals, a second pair of peak rectiers, means for applying low frequency sawtooth wave form signals to said second pair of peak rectiers with like polarity at each rectifier, means including a low pass filter for applying low frequency pulse signals to said second pair of rectiers from said transformer with opposite polarity at each rectifier, and means for utilizing the output of said second pair of'rectifiers for controlling said means for applying low frequency sawtooth Wave form signals.

GORDON L. FREDENDALL.

REFERENCES CITED Thev following references are of record in the file of this patent:

UNITED STATES PATENTS 

